1. Field of the Invention
Embodiments of the invention generally relate to a valve actuator. More particularly, embodiments of the present invention relate to a mechanical override for a valve actuator.
2. Description of the Related Art
Various designs of valve actuators exist that operate to open and close valves in a variety of uses. The petroleum industry utilizes these actuators to operate gate valves that incorporate a sliding gate within a valve body to selectively block fluid flow through tubing. Positioning gate valves along tubing at various locations controls and directs the flow of fluids through the tubing. An assembly known as a Christmas tree includes these valves along with spools, pressure gauges, fittings, and/or chokes connected to the top of a well in order to direct and control the flow of formation fluids or production fluids from the well.
In operation, a valve stem extending from the valve body of the gate valve moves axially within the valve body in order to move the sliding gate between an open position and a closed position. A shaft within the actuator for the gate valve engages the valve stem to impart the axial movement to the valve stem. Typically, the actuator includes a spring to bias the shaft within the actuator such that the gate valve provides a failsafe to the closed position. In this manner, force applied to the shaft of the actuator from either a hydraulic, pneumatic, or mechanical source, depending on the type of actuator, overcomes the bias of the spring to move the sliding gate to the open position. Other gate valve designs provide for the failsafe in the open position or a fail-in-position which maintains the position of the sliding gate upon failure. If the shaft or a top shaft coupled to the shaft extends external to the actuator, the amount of the shaft extending from the actuator serves as a visual indication as to whether the valve is in the open position or the closed position.
The actuators commonly used to open and close the gate valves include manual actuators that use a mechanical force, automatic actuators such as various designs of pneumatic or hydraulic actuators, or combination actuators having both manual and automatic operation. Since most automatic operations of the actuators have a maximum capability for applying force to the valve stem, the combination actuator permits additional opening/closing power on a temporary basis without having to remove the original automatic actuator. Inadvertent loss of pneumatic or hydraulic pressure to the automatic actuator closes the gate valve which can interrupt production and interfere with wellbore completion operations occurring through the valve. The combination actuator provides a back-up for automatic operation, allows for testing, and enables an operator to lock the valve in the open position during various wellbore completion operations.
The shaft extending outside of the actuator provides the ability to lock open the valve with the use of a cap. Originally, the cap was a solid cap threaded to an exterior portion of the actuator such that the cap interfered with the axial movement of the actuator's shaft in order to hold the valve in the open position. However, the solid cap overrides the failsafe to the closed position. A fusible portion in a bore of the cap was added so that the fusible portion in the event of a fire burns out and permits the shaft to travel axially through the bore of the cap. While the fusible portion solves safety concerns in the event of fire, the cap still prevents automatic operation of an automatic actuator without removing the cap. Thus, the cap defeats the purpose of paying for and having the benefits of the automatic operation provided by the automatic actuator. Further, the valve must be forced into the open position before the cap can be installed on the outside of the actuator.
One type of combination actuator couples an auxiliary accessory or mechanical override such as a manual handwheel assembly to an exterior portion of an automatic actuator. The handwheel assembly couples to the combination actuator and mechanically forces the shaft of the actuator axially against the bias of the actuator's spring. Typically, the handwheel assembly includes an internally threaded bore that a threaded shaft threads into and contacts the shaft of the actuator. Thus, rotation of the threaded shaft axial moves the threaded shaft through the threaded bore and transposes movement to the shaft of the actuator. However, this type of handwheel assembly overrides the failsafe of the actuator similar to the cap. Additionally, the only visual indication of the position of the valve occurs when the threaded shaft of the handwheel assembly is rotated inward to overcome the bias of the actuator spring since the threaded shaft must be extended fully outward in order to allow for the automatic operation of the actuator. Thus, there is no visual indication of the position of the sliding gate during automatic operation. Furthermore, thieves easily remove the auxiliary accessories such as the handwheel assembly since the accessories thread to the exterior of the actuator.
A second type of combination actuator having an integral mechanical override includes an elongated exteriorly threaded valve stem of the gate valve that threads into an elongated internally threaded bore in the shaft of the actuator. In this manner, the mechanical override is integral with the actuator unlike the auxiliary accessories that attach to the exterior of the actuator. The shaft of the actuator extends through the actuator to an outside of the actuator where a handwheel attaches to the shaft for applying rotation to the shaft. Thus, manual rotation of the shaft provides relative axial movement between the shaft of the actuator and the valve stem due to mechanical threading/unthreading of the valve stem within the threaded bore of the shaft. Since the shaft of the actuator remains axially stationary, the relative axial movement provided by the manual operation moves the sliding gate between the open position and the closed position. In an automatic operation, the entire shaft of the actuator axially moves in order to move the sliding gate between the open position and the closed position. The position of the actuator's shaft remains the same for the automatic operation in a non-actuated position and any position of the manual operation of the actuator. Thus, there is no visual indication as to the position of the sliding gate unless the actuator is in an actuated position during automatic operation. Additionally, the integral mechanical override overrides the failsafe of the actuator similar to the cap and the handwheel assembly.
There are further problems associated with current integral mechanical overrides. The valve stem has a small diameter due to size constraints of the valve body and the actuator. The small diameter of the valve stem along with V-threads on the threaded valve stem and bore, make the shaft difficult to rotate. Thus, current integral mechanical overrides allow well fluids and thereby debris to enter the thread area of the valve stem and bore in order to pressure assist the rotation of the shaft during the manual operation. This fouls the threads and reduces the safety of the actuator since well fluids enter portions of the actuator. To further ease rotation of the shaft, the threaded bore and the valve stem have a large number of threads per inch, thereby requiring more manual work to manually operate the actuator. Additionally, the threaded valve stem and the bore must be elongated since the diameter of the valve stem is small and the V-threads do not function well under loads. The elongation takes up valuable space in the actuator.
Thus, there exists a need for an improved mechanical override for use with an actuator for a valve. There exists a further need for a mechanical override that provides a visual indication of the position of the valve, permits the actuator to maintain a failsafe position, improves and isolates a drive thread of the mechanical override from well fluid, and couples integrally with the actuator.